Satellite based telecommunications systems have been proposed for enabling user terminals to communicate with one another and with an existing public telephone switching network (PSTN). The user terminals interconnect with one another and with the public telephone network through earth stations strategically located at predefined geographic positions about the earth.
The proposed systems utilize a constellation of telecommunications satellites for relaying communications signals between the user terminals and earth stations. These communications signals pass along predefined channels uniquely assigned to each user terminal Each channel includes a forward link along which an earth station transmits RF signals to a mobile terminal and a return link along which the user terminal transmits RF signals to the earth station. Each communications link passes through a corresponding satellite which functions as a "bent pipe" and retransmits all received communications to the corresponding earth station or user terminal.
Typically, satellite based telecommunications systems utilize one or more coding techniques to enhance the system capacity. For instance, the system may employ frequency division multiple access (FDMA) coding, time division multiple access (TDMA) coding, code division multiple access (CDMA) coding, or any combination of FDMA, TDMA, and CDMA coding. In general, to maximize system capacity, the earth station synchronizes in frequency and timing each return link assigned thereto in order to prevent interference between transmissions emitted from different user terminals. To maintain synchronization, the transmissions from multiple user terminals must be emitted at predefined times to ensure that the transmissions are received by a common earth station simultaneously Variations in timing result as the distance varies between a coverage satellite and a transmitting user terminal. By way of example, as the distance increases between a satellite and transmitting user terminal, the transmission arrives later in time at the earth station. Divergently, as the distance decreases between a satellite and transmitting user terminal, the transmission arrives earlier in time. To account for such range variations, the user terminals are controlled to retard and advance the starting times for data transmissions (i.e, frames of communications data) to ensure that frames from multiple user terminals assigned to a common earth station arrive at the earth station simultaneously.
In addition, each earth station controls user terminals assigned thereto in order to ensure that the return link remains centered about an assigned carrier frequency (i.e., sub-band) as received at the earth station. The center frequency of each transmission from a user terminal experiences frequency changes due to the Doppler effect. The Doppler effect occurs due to the fact that a satellite continuously moves relative to a transmitting user terminal. The perceived carrier frequency of the receiver increases as the satellite moves toward a user terminal and decreases as the satellite moves away from the user terminal. If not corrected, Doppler induced frequency shifts and timing misalignment create co-channel interference between transmissions received from multiple user terminals. Thus, the user terminals continuously adjust the carrier frequency and timing of outgoing transmissions to ensure that the perceived carrier frequency and timing at the assigned earth station receiver equals the assigned carrier frequency and timing for the user terminal. A need remains for accurately maintaining the forward and. return links between an earth station and user terminal.
Moreover, a need remains to facilitate handovers. Throughout transmission, satellites continuously orbit the earth. The user terminals may also move. Consequently, a communications link with a user terminal may need to be transferred or handed over from one earth station to another earth station. When a handover occurs, the user terminal must be changed to a new channel in which it establishes new forward and return links between the user terminal and the new earth station. This adjustment includes changing the timing of the user terminal to align with the new earth station's reference time. The user terminal transmitter must also shift to a new carrier frequency. A need exists to facilitate quick and reliable handovers by minimizing the time needed to achieve synchronized forward and return links with the new earth station.
Moreover, past systems have been unable to provide earth station sharing wherein multiple earth stations communicate with a user terminal at the same time through a common satellite. Earth station sharing requires timing and frequency alignment at the common user terminal of RF signals from both earth stations. Such alignment is complicated due to the presence of differing earth station to satellite path links and Doppler variations. Also, existing systems have been unable to provide return link synchronization with earth station sharing in which user terminals commmunicate in a common subband through a common satellite to multiple earth stations. To do so, the received communications signals must be aligned in time and frequency at any earth station receiving such signals.
A need remains within the industry for an improved telecommunications system capable of maintaining synchronization between multiple user terminals and sharing earth stations. It is an object of the present invention to meet this need.